Highly radioactive wastes, such as fission-product solutions resulting from processing irradiated nuclear fuels, are converted to solid, stable and substantially water-insoluble products before they are transported to a final storage location in conformance with environmental protection requirements. Various methods are used to solidify highly radioactive liquids, salts or ashes. Liquids are initially processed to form dry residues and are calcined, for example in a spray calciner. The resulting product is mixed with vitrifying substances, melted and filled into a mold for tempering and slow cooling. The calcined, radioactive products are also mixed with substances with which they form, under similar treatment, a ceramic mass or a basaltic mass. The solid molded bodies are then provided with a coating and are stored in closed or abandoned salt mines or in salt caverns built for this purpose. Uncontrolled return of radionuclides into the biocycle is thus made practically impossible.
When cylindrical glass blocks are produced with a diameter of 200 millimeters (mm) and a length of 800 mm, i.e. a volume of approximately 25 cubic decimeters (dm.sup.3), a heat of decomposition of 1 kilowatt (kw) per block, i.e. 40 w/dm.sup.3 of glass (resulting from incorporated radionuclides) is permissible since this corresponds to a surface temperature of about 400.degree.C for a block stored in air with free convection. The heat output of 1 kw corresponds, for example, to 3.51 kilograms (kg) of a fission-product mixture whose incorporation into the glass block was effected after one year of decay. If a larger quantity of such a fission product mixture were incorporated in one block, the inherent heat would be greater and would unfavorably influence storage in salt. Due to higher surface temperature, the block would render the surrounding salt layer plastic and would thus slowly sink into the salt.
For storing the molded bodies in salt, bores are made in the salt into which the molded bodies are inserted. The quantity of heat transferred from the molded bodies to the salt dictates a certain minimum spacing between bore holes. With a heat output of 1 kw per molded body such minimum spacing is about 10 meters (m).
Solidification of radioactive waste, particularly highly radioactive waste, as well as its transport to an ultimate storage location is very expensive. This expense cannot be avoided, however, in view of the need to protect the environment, and the need for extensive storage facilities. In a circular area with a diameter of about 20 m it is possible to make a maximum of only seven bore holes for the above-noted reasons; this is very unsatisfactory. When projected quantities of fission products are considered, the burden imposed by the lack of sufficient storage facilities is readily apparent. The development of energy generation from nuclear reactors in the Federal Republic of Germany is estimated to reach 20 gigawatt electrical (Gw.sub.el) for thermal reactors by 1980. This would result in a total fission product quantity of 15.8 tons per year for 1980 and the following years (assuming there is no further increase). Were these fission products to be incorporated in, e.g., a glass mass, a total volume of glass blocks of 111 m.sup.3 per year would result.